When a container has no labels, Docker returns {"Labels": null} in the
inspect response. The code was using .get("Labels", {}) which only
returns the default when the key is missing, not when it's null.
This caused _has_webterm_label() to raise TypeError, which was silently
caught by the event watcher's exception handler, causing it to reconnect
and miss the container start event.
Fixed by using .get("Labels") or {} pattern in:
- _handle_event() when processing start events
- _get_container_command() when extracting command label
- _get_container_theme() when extracting theme label
Added test for null labels case.
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